The volume sector of Structural Composites is dominated by Unsaturated Polyester Resins (UPR) reinforced with glass-fibre, and commonly known as GRP (Glass-fibre Reinforced Polyester). The reactive diluent for the resin is invariably styrene, which is becoming subject to more stringent regulatory measures, particularly lower occupational exposure limits in the workplace. This has led resin suppliers and processors to seek alternative solutions to styrene. A first approach has involved the use of alternative vinyl aromatic monomers such as vinyl toluene and divinyl benzene. However, the regulatory status of these substances is not drastically different than that of styrene. A second approach has been the use of methacrylate monomers typified by butanediol dimethacrylate, triethylene glycol dimethacrylate and hydroxypropyl methacrylate. Lower vapour pressures of these methacrylates means that emissions into the workplace atmosphere are much lower than those of styrene. However, this advantage is offset by the potential skin sensitising properties of these substances.
There are many different conversion processes for the production of Structural Composite parts. The method chosen depends on a variety of factors such as: number of parts to be produced, size of part, structural requirement and desired production rate. High production rates can be achieved with hot press moulding where the resin is first converted into a ‘pre-preg’: a resin pre-impregnated reinforcement. In the art, pre-preg is a term for “pre-impregnated” composite fibres. For hot press moulding these pre-pregs are typified by Dough Moulding Compound (DMC) and Sheet Moulding Compound (SMC).
WO9222603 (DSM) discloses a semi-finished product consisting of fibrous material and a virtually powdered thermosetting resin having a glass transition temperature above 35° C., a curing temperature at least 40° C. higher than the glass transition temperature, and a viscosity of less than 5000 Pa·s at a temperature between the glass transition temperature and the curing temperature, the resin particles being partly fused and partly in contact with the fibrous material. All examples rely on a single composition based on unsaturated polyester, however they require the use of at least one diluent, namely triallylcyanurate (a high-boiling copolymerisable monomer); curing was performed at 150° C.
Thus, there is a need for a monomer-free composition that can be cured at a temperature as low as 80° C.
The subject of the current invention is a novel pre-preg concept using a powder resin thereby drastically reducing volatile workplace emissions of substances of potential concern and potential skin sensitisation issues. Additionally the invention has significant processing and productivity benefits.